1. Field of the Invention
The present invention relates to a method of manufacturing an aseptic soya bean curd without using a germicide.
2. Description of the Prior Art
Recently, a new coagulating agent, glucono-delta-lactone (hereinafter abbreviated as GDL) has been developed. Using this agent, new methods of manufacturing soya bean curd have also been developed. For example, a packed soya bean curd manufactured by putting soya bean juice and GDL into a bag or injection molded container, sealing the container and heating the container to coagulate the contents is now commercially available.
According to this method, a soya bean curd can be manufactured easily and on a mass production scale. The product is very portable and fairly good in its preservation qualities. Therefore, this method has given rise to a new breadth in the distribution of these products.
However, an aseptic soya bean curd which can be preserved for a long period of time cannot be manufactured by this conventional mass production manufacturing method. In the step of soaking the beans and grinding them to obtain a crude juice of the soya bean (the so-called "Go", in Japan) and then boiling the crude juice by heating and filtering it to obtain a soya bean juice, the heating is conducted generally at about 100.degree. C for 4 to 8 minutes.
Under such heating conditions, although general bacteria can be killed, spore-forming bacteria originating from the soya bean cannot be killed. Consequently, several tens or several hundreds of spore-forming bacteria generally exist in a 1 ml sample of soya bean juice treated by such heating. These spore-forming bacteria survive to exist in the soya bean curd which is the final product. As a result, the subsequent manufacturing steps must be strictly controlled, and the product deteriorates during transportation and preservation.
Therefore, in order to obtain an aseptic soya bean milk, as in the method described in Japanese Patent Publication Gazette No. 34224/73, a soya bean protein solution (the so-called "soya bean milk") is introduced into a pipeline together with steam and heated to above 120.degree. C.
Regarding the coagulating agent solution to be mixed with the soya bean juice, heretofore, the sterilization of the coagulating agent, for example, GDL solution, used in the manufacture of the soya bean curd has not been performed. Additionally, an indispensable requirement for producing a true aseptic soya bean curd is to prevent the secondary contamination of the soya bean milk together with the coagulating agent during the time of filling into a packaging container. When soya bean milk is filled into a conventional container, the product itself cannot actually be aseptic because the bacteria in the air are drawn therein. In any event, since the GDL solution is not itself sterilized, a conventional container is used and the soya bean milk is treated using conventional filling methods. The final product has poor stability, which would be true even if the soya bean milk could be sterilized. As a result, it is impossible to form a high quality product which can be preserved at room temperature for a long period of time.
Given the state of the art, synthetic germicides, such as AF.sub.2 and the like, have been used in the products. However, recently, the toxicity of synthetic germicides, including AF.sub.2, has come into question. As a result, a method of manufacturing soya bean curd which can be preserved for a long period of time without requiring the addition of a synthetic germicide is needed.
In summary, in the conventional methods of manufacturing soya bean curd packaged in a blow molded container, a true aseptic soya bean curd has not been obtained because blow molded containers cannot be made aseptic, the soya bean juice cannot be filled into the container in an aseptic atmosphere, the coagulating agent solution to be added to the soya bean juice is not aseptically treated, and the soya bean juice is not sterilized.
Improved methods of filling a soya bean juice into a container and pasteurizing a coagulating agent solution have been proposed (Japanese Patent Public Disclosure No. 85758/73). However, a true aseptic soya bean curd in a blow molded container cannot be obtained by this method because the coagulating agent solution and soya bean juice are not sterilized and because no concrete means of making the aseptic container is described. Some methods of sterilizing the soya bean juice have been proposed (Japanese Patent Publication Gazette Nos. 34224/73 and 34225/73). However, even if the soya bean juice is sterilized according to these methods, an aseptic soya bean curd in a blow molded container cannot be manufactured, since the container cannot be blow molded and the soya bean milk cannot be filled into the container in an aseptic atmosphere.
On the other hand, methods of sterilizing cow's milk and filling it into a container aseptically are widely carried out in the milk industry. However, application of the cow's milk treating steps to the manufacture of soya bean curd for producing an aseptic soya bean curd has not hitherto been tried and is considered to be impossible. That is, it is well known in the milk industry that the container filled-up with the cow's milk in an aseptic atmosphere is cooled as soon as possible after being sealed tightly. There is no technical knowledge on the concept of heating this container. Therefore, application of the steps used in the treatment of cow's milk to the manufacture of soya bean curd has not hitherto been tried since it is an indispensable requirement in the manufacturing of soya bean curd to heat the soya bean juice to which has been added a coagulating agent in order to coagulate the soya bean juice.
Moreover, mixing of two solutions in a pipeline has hitherto been widely carried out in the industrial field. Also, in the food industry methods using various pipeline mixers have been proposed ("Mixing and Stirring in Food Industry", by Kazuo Endo, published from Korin Shoin, March 10, 1963, Japan). Additionally, a method of mixing specific quantities of water with a sugar solution is described in the February issue of Machine and Apparatus for Food, pages 61 to 72, published in Japan (1974). However, it is sanitarily insufficient to use such mixing in a pipeline alone in the manufacture of aseptic foodstuffs. The manufacture of the desired aseptic foodstuff is not possible unless the steps before and after the mixing are carried out in an aseptic atmosphere. But, of course, in the manufacture of foodstuffs, in particular soya bean curd, it is very difficult to carry out the whole process in an aseptic atmosphere.
The utilization of a pipeline for mixing soya bean juice and a coagulant-containing solution in the manufacture of soya bean curd has not hitherto been tried. Certainly, no attempts at utilizing such a treating method for continuously manufacturing an aseptic soya been curd on a large scale have been made.
Moreover, in the manufacturing of a packaged soya bean curd in general, there are such major difficulties that the process cannot be carried out continuously in its entirety because the soya bean juice and coagulant-containing solution are poured into a tank and must be stirred for a specific length of time to ensure homogeneous mixing.
The present inventors have responded to this need for a method of manufacturing an aseptic soya bean curd packaged in a container, in which the whole process can be carried out continuously and in which the above-described defects are eliminated.